DNA Synthesis 149 



labelled. The only labelled cells would be those that move out of the 

 villi after a lapse of time of about 6 to 8 hours. Then the level at which 

 we get the autoradiograph reaction would be about 6 or 8 hours behind 

 what it should be. And that would explain the discrepancy between the 

 figure we obtained with the autoradiographic method and with the 

 other two methods. 



Popjak: Do you know what happens to the other phosphate com- 

 pounds during mitosis? Does, for example, inorganic phosphate 

 penetrate the cell or cell nucleus during mitosis? 



Howard: I don't know. Our work hasn't covered any phosphorus 

 compounds other than DNA, and we don't know at all from our own 

 work in what form the phosphorus goes into the nucleus. 



Popjak: There is a possibility that during mitosis the cell nucleus 

 maintains a peculiar autonomy; i.e. that it metabolizes only those 

 substances which it has accumulated before, and that there is actually 

 no penetration of phosphorus into the nucleus during mitosis. 



Howard: We don't have any uptake in the DNA fraction during the 

 division. 



Pelc: There is some indication about that point in our tissue culture 

 experiments. After extraction with 80 per cent alcohol only (which 

 takes out most of the inorganic phosphorus but still leaves quite a lot 

 of other compounds, such as the nucleic acids) we can see that some of 

 the nuclei do not contain phosphorus, while others do. It seems from 

 our experiments that this autonomy of the nucleus would have to start 

 some hours before visible prophase. 



Leblond: This seems to be an extremely important point. Since 

 the earliest work of Hevesy there has been a discrepancy between the 

 percentage of DNA present in the nucleus and the rate of mitosis. In 

 organs like the intestinal mucosa and the spleen the discrepancy is 

 small, but in the case of the liver the discrepancy is large. To give you 

 the precise figure, in the adult rat liver the percentage of cells that go 

 into mitosis per day has been calculated in our laboratory to be about 

 0- 1 per cent of the cells, while the renewal rate, calculated on the basis 

 of DNA, is of the order of 1 per cent, and in some authors' work, up to 

 2 per cent and more. This discrepancy has not been explained, and the 

 situation has been further complicated recently by the study of the 

 incorporation of substances other than phosphorus into the DNA. 

 Thus, the work of Brown and associates indicated that the entry of 

 adenine into the nucleoprotein corresponds far more accurately to the 

 true rate of mitosis (about • 1 per" cent renewal per day). With 

 [i^N] glycine Reichard found values of about 0-7 per cent per day, 

 similar to those obtained with radiophosphorus. Finally, with [^^C] 

 glycine (LePage and Heidelberger) the incorporation is so great that 

 calculations of renewal rate make no sense. The type of explanation 

 that has been proposed is that some parts of the molecule may be 

 renewed either by exchange or true chemical processes, but that the 

 bulk of the molecule is not. 



Brown: In regard to the point about adenine, in our original work 

 with adenine it was shown that there is little incorporation of adenine 



